Burners of various kinds and strengths combust outside air and supplied fuel to produce heat. Adding heat to a building often dries the remaining air inside a building. In drying the building air, moisture can be extracted from a building as during remediation from a flood, a fire suppression event, and cleaning activities to name a few. The water dryout industry has long held the erroneous premise that direct gas fired heaters should not be utilized for dryout of flooded buildings because of the heaters adding moisture into the air within a building as a residual from the combustion process. Although moisture is released as part of the combustion process, the amount of water created is relatively small when compared to the volume of dilution air that is provided with a direct fired heater. The combustion process produces 0.095 pounds of water per cubic foot of natural gas. So, the opponents against utilizing direct fire heaters for dryout applications focus on the 95 pounds of water that a 1 million Btu per hour heater produces every hour.
Within the dryout industry, opponents of heater usage overlook the dilution aspect of the fresh outside air being supplied by the direct fired heater. A one million Btu/hr heater operating at a 140° F. temperature rise and delivering 6,000 cfm will convey over 27,000 pound of air (6000 cfm×60 min/hr×0.075 lb/cf (air density handled by the blower)) while producing the 95 pounds of water vapor. This equates to 0.0035 pounds of water per pound of dry air and when added to the moisture present in the fresh outdoor air, the heated discharge air has typically less than 2% relative humidity, or RH, and thus is very dry or almost desert like.
As an example of the limited amount of water vapor added by combustion to air within a building, consider that outdoor air at 40° F. and 60% RH has an air density of 0.0794 pounds per cubic foot and a moisture content of 0.00314 pounds of moisture per pound of dry air or 22 grains of moisture per pound of dry air. When this outdoor air is heated to a discharge temperature 180° F. following combustion, the fuel gas consumed by combustion will be 1,047,638 Btu/hr (from 6000 cfm×0.0794 lb/cf×0.241×60 min/hr×140/0.92) resulting in 99.5 pounds of water vapor. This equates to 0.0035 pounds of moisture per pound of dry air (from 99.5/(6,000×60×0.0794)) or 24.4 grains of moisture per pound of dry air. The fresh outside air heated to 180° F. and delivered to the building being treated contains 0.0066 pounds of moisture per pound of dry air or 46.4 grains of moisture per pound of dry air. From a high temperature psychometric chart, this point of combustion, 180° F. at 0.0066 pounds of moisture per pound of dry air, indicates a relative humidity of below 2%.
Direct gas-fired industrial air heaters are used extensively to provide replacement air to match air that is exhausted or to provide ventilation air in industrial and commercial occupancies. These heaters typically operate around the clock, year round, and it is therefore important to minimize the temperature rise of these heaters during mild weather operation so as not to overheat the space. With the airflow held constant as is the case with most make-up air heater applications, the minimum temperature rise relates to the minimum gas flow rate.
For burner systems which ignite a pilot light and establish a proper flame signal for the pilot prior to energizing the main burner gas valves, the ignition of the main burner gas is readily accomplished even at the minimum fire condition. In the industry this type of ignition system is referred to as an “intermittent pilot ignition system.” These systems have generally required only one input for supervising or monitoring the presence of flame and that sensor is typically located in close proximity to the pilot flame so as to sense its presence. In some ignition systems, gas flow to the pilot burner would be shut off after adequate time had expired for establishing the main burner flame, thereby having the flame sense circuit actually sense the main burner flame once the pilot flame had extinguished itself. This type of ignition system is referred to as an “interrupted pilot ignition system.”
Direct ignition systems are another means for lighting the main burner gas. However, the present invention omits a pilot system. Ignition of the main burner occurs immediately after the main gas valve is energized. There is a variation of this type of ignition system which may be referred to as a “proven source” type of direct ignition system where current flow to the ignition device is confirmed to be functioning properly prior to opening the main burner gas valve. All of the above ignition systems have functioned with equal reliability for many years in millions of different heating appliances.
A properly designed direct ignition system in a direct gas-fired industrial air heater or make-up air heater application is most difficult or challenging from an engineering standpoint because this system must ignite the main burner over an extremely wide range of gas flow rates. To contemplate this aspect of the application challenge in a more detailed manner, one needs to understand that the ignition source, whether it is a high voltage spark or a hot surface ignition device, is generally only present for a few seconds and can be extremely small with respect to the size of burner that it is being utilized on. Gas flow must reach the area of the burner where the ignition source is located with the proper fuel to air ratio to obtain ignition.
During the development of the Harmonized Standard for Direct Gas-Fired Industrial Air Heaters between the United States and Canada, a provision was added that required the main burner flame supervision means for burners over 36 inches in length to be as remote as possible from the ignition source to ensure flame propagation has occurred and is maintained over the entire length of burner. To accommodate this requirement in pilot ignition type systems, a second flame detection device can been employed along with the associated controls which switches the pilot sensing system to the main burner flame sense controls after a preset time delay which allows for the flame to propagate across the burner length.
The impact of this provision cause more problems for direct ignition systems with regard to ignition at the minimum fire condition and the time required for that small flame to propagate across the full length of the burner. The flame establishment time period typically only last for only a few seconds after energizing the main gas shut-off valves. The ANSI standard limits the flame establishing time period to a maximum of 15 seconds for direct ignition systems with burners over rated 400,000 Btu/hr and thus, the manufacturer would desire to keep this time as short as possible. Direct fired heaters are not vented and in the case of a delayed or failed ignition, raw gas is dumped into the space being heated. Though the actual quantity of gas may be small and not pose an unsafe condition for the building or its occupants, the noticeable odor from the gas, mercaptan, may unnecessarily incite an adverse reaction to the occupants of a building.
Without one of the control methodology provided as the basis for this invention, the minimum gas flow adjustment would have to be significantly increased or other more expensive gas flow controls systems is employed for direct ignition type systems to ensure that the flame would propagate across the burner within the flame establishment time period. Longer burners would require a higher minimum fire adjustment to account for the distance that the flame has to travel. Increasing the minimum gas flow rate also increases the minimum temperature which then unfortunately overheats the conditioned space during mild weather.